At present, the technology employed in these plants involves the use of multistage concentrators, each of them including a heat exchanger, a flash drum and a circulating pump. The liquid to be concentrated is sent by the pump to the heat exchanger where it undergoes a predetermined positive increment in temperature, then it goes into the flash drum where, owing to the presence of a lower pressure than in the exchanger, it evaporates and there is consequently production of steam which in turn is used to heat the next stage, also including a heat exchanger, a flash drum and a pump and so on, up to a certain number of stages until the steam produced in the last stage is sent to a condenser which can be either of the mixture type or of the surface type.
The first stage is heated by steam coming directly from the system while the product to be concentrated goes from one stage to the next either by means of pumps or, less commonly, owing to the difference in pressures existing between one stage and the next.
In these plants, as described above, there are as many circulating pumps as stages; this means that a multistage plant can only have a limited number of stages, since the amount of steam produced by the boiler and used to operate the turbine which in turn actuates the product circulating pumps must be such that it can be completely absorbed by the first stage.
Therefore, if the number of stages is to be increased, it is necessary to supply additional circulating pumps powered by additional electric motors. This involves greater energy requirements.